1. Field of the Invention
The present invention relates to a process for making improved net shape or near net shape metal parts using powder metallurgy. More specifically, the process of the present invention sinters a compact containing a metal and chemically-bound oxygen in the form of a metal oxide powder in an amount sufficient to improve the sintering process. The chemically-bound oxygen is reduced during the sintering process which takes place in a reducing atmosphere. In addition, the compact can contain a metal oxide and a solution compound to produce a metal alloy part. In a preferred embodiment, the compact contains particles of a reinforcement compound having a melting point higher than the metal in the compact.
2. Description of Related Art
Advanced metal alloys and metal matrix composites have been produced for many years using conventional powder metallurgy procedures. With oxidized metals being undesirable in conventional sintering processes, metal powders that contain no more than about 0.1 to 1.0% by weight oxygen are used to make compacts prior to sintering. Conventional metallurgical processes include reduction prior to compaction and sintering to ensure a minimum level of oxygen in the system.
Green compacts can be made by various forming technologies which include extrusion, roll compaction, cold isostatic pressing, dry pressing and metal injection molding. The dimensions of the green compact are determined by the size of the die, taking into account the dimensions of the finished part and the shrinkage during sintering. In most cases, the powder is compacted into green parts using conventional uniaxial mechanical or hydraulic dry presses at a pressure of 45,000 to 55,000 psi. Extrusion forming is used for rods, tubes and sheaths having a fixed cross section. Roll compaction is advantageously used for large, flat and thin substrates. Cold isostatic pressing is preferred for large parts, whereas, dry pressing is more economically feasible for small, simple geometric parts which must be produced in large volumes. Metal injection molding which utilizes one of a variety of binders can economically produce parts of complex geometry in large volumes. The microelectronics and diode industries, for example, require a large-volume production of metal parts having tight tolerances for use in high reliability applications.
An important objective in the production of advanced alloys or metal matrix composites using powder metallurgy technology is to produce sintered parts in which the fired density approaches theoretical. Depending on the metal system employed, there is a significant difference in the physical properties of parts as their densities increase from about 96% to 99.5% of theoretical. For example, parts made of 4640 steel have an ultimate tensile strength which ranges between about 700 to 990 Mpa, a tensile yield which ranges between about 630 to 900 Mpa, and tensile elongation which ranges between about 2% and 15% at densities between 96% and 99.5% of theoretical, respectively. See German, Randall M., Powder Metallurgy Science, Metal Powder Industries Federation, 2nd Edition, 1994, p. 307. In conventional technologies, key variables affecting density for powder metallurgy processing, include the sintering temperature, the compaction pressure and the particle size of the powder. Other groups of researchers have found that it is important during the powder metallurgy process to minimize the amounts of oxygen and impurities in the system. German, Randall M., Powder Metallurgy Science, p. 307.
Metal matrix composites, in particular, provide design flexibility since key properties may be tailored to meet design specifications. In addition, metal matrix composites are synergistic in that the lower melting metal enables material transport within the compact at lower processing temperatures, whereas the higher melting reinforcement compound provides improved physical properties of the finished product. However, it is commonly known that a main disadvantage associated with composites is that the conventional processes for making them are complicated and yield low density parts. For example, in the case of low cost steel systems a density greater than 85% of theoretical is difficult to achieve.
The production of metal matrix composite parts which have consistent material ratios is important. As an example, copper/tungsten and copper/molybdenum composites are widely used in various electronic applications due to their relatively high thermal conductivities of 150-250 W/mK. Moreover, because the coefficient of thermal expansion of the composites can be controlled by varying their Cu/W and Cu/Mo ratios, these composites find significant use in electronic packaging applications where tailoring the composite to match the thermal expansion characteristics of the chip or other device attached thereto is highly desired.
Metal matrix composite parts having high-density/specific gravity and low porosity are also desirable in certain applications. For example, composites having high specific gravity can be used as counterweights in sport equipment, automotive parts and many other mechanical applications.
Metal matrix composites can be made by a number of techniques. In one technique, known as infiltration, a shaped article formed from a sintered mass of reinforcement particles is contacted with molten metal. As a result, the molten metal is infiltrated into the voids and interstices between the sintered reinforcement particles, thereby forming a completed composite.
In another technique, a powdery mixture of copper oxide particles and tungsten oxide particles is reduced in a dry (the dew point is less than -40.degree. C.) hydrogen atmosphere. The reduced powder is mixed with or without a binder, and the mixture is compacted and sintered. Additional metal can be added by infiltration if desired. See U.S. Pat. No. 3,382,066 to Kenney et al., the disclosure of which is incorporated herein by reference.
A similar technique is illustrated in U.S. Pat. No. 5,439,638 to Houck et al., the disclosure of which is also incorporated by reference. In this technique, a mixture of reinforcement particles such as tungsten powder, metal oxides such as copper oxide powder, and optionally, cobalt powder is milled in an aqueous medium to form a slurry. The liquid is removed from the slurry via a spray dry process to form spherical, flowable agglomerates. The agglomerates are subjected to a reducing atmosphere to form a flowable tungsten/copper composite powder, and the powder then compacted and sintered to form the copper/tungsten composite.
There are many disadvantages to the above infiltration and co-reduction of oxide powder processes. For example, infiltration processes are generally unable to produce net shape parts. Parts produced by infiltration must either be pressed again via a second compaction process or be machined into final shape, thereby greatly increasing complexity of manufacture and cost. Also, typical infiltration processes require the extra steps of binder burn-off and pre-sintering. Moreover, in such processes, the pre-sintered compact is often relatively friable, which may also result in part breakage and a lower yield. Also, during the infiltration process, which is typically carried out in a separate furnace, excess metal may form pools or bleedout, resulting in the production of defective parts which must be discarded, or at least subjected to extra machining after sintering. Infiltration may also require special fixtures and complicated furnace equipment. Processes involving co-reduction of oxide powders also involve extra processing steps and are, hence, inherently complex. Also, machining after firing is still necessary in many instances. Because of these complexities and disadvantages, commercial manufacture of high quality, metal matrix composites is still difficult to achieve consistently and economically in large volumes.
Accordingly, there is a need for a new process for producing advanced metal alloy parts and metal composite parts, which is easier and less expensive to carry out than prior art processes and which is capable of producing parts with densities of about 97% and preferably about 99.5% of theoretical, rapidly and consistently.